This invention relates to image projection displays and more particularly to a color wheel synchronization technique employed in an optical pathway of such displays.
Projection systems have been used for many years to project motion pictures and still photographs onto screens for viewing. More recently, presentations using multimedia projection systems have become popular for conducting sales demonstrations, business meetings, and classroom instruction.
In common operating mode, multimedia projection systems receive analog video signals from a personal computer (xe2x80x9cPCxe2x80x9d). The video signals may represent still, partial-, or full-motion display images of a type rendered by the PC. The analog video signals are typically converted in the projection system into digital video signals that control a digitally driven image-forming device, such as a liquid crystal display (xe2x80x9cLCDxe2x80x9d) or a digital micro mirror device (xe2x80x9cDMDxe2x80x9d).
A popular type of multimedia projection system employs a light source and optical path components upstream and downstream of the image-forming device to project the image onto a display screen. An example of a DMD-based multimedia projector is the model LP420 manufactured by In Focus Systems, Inc., of Wilsonville, Oreg., the assignee of this application.
Significant effort has been invested into developing projectors producing bright, high-quality, color images. However, the optical performance of conventional projectors is often less than satisfactory. For example, suitable projected image brightness is difficult to achieve, especially when using compact portable color projectors in a well-lighted room.
Because LCD displays have significant light attenuation and triple path color light paths are heavy and bulky, portable multimedia projectors typically employ DMD displays in a single light path configuration. Producing a projected color image with this configuration typically requires projecting a frame sequential image through some form of sequential color modular, such as a color wheel.
The use of color wheels in frame sequential color (xe2x80x9cFSCxe2x80x9d) display systems has been known for many years and was made famous (or infamous) in early attempts to develop color television sets. With technological advances; however, color wheel display implementations are still useful today.
FIG. 1 shows a typical prior art FSC display system 10 in which a sensor 12 senses a timing mark 14 to detect a predetermined color index position of a motor 16 that rotates a color wheel 18 having respective red, green, and blue filter segments R, G, and B. A light source 20 projects a light beam 22 through color wheel 18 and a relay lens 24 onto a display device 26, such as an LCD-based light valve or a DMD. A display controller (not shown) drives display device 26 with sequential red, green, and blue image data that are timed to coincide with the propagation of light beam 22 through the respective filter segments R, G, and B of color wheel 18. Clearly, successful operation of a FSC display system depends on properly synchronizing the red, green, and blue image data to the angular position of color wheel 18.
Sensor 12 typically employs any of optoelectrical or electro mechanical shaft position or motor armature position detectors and usually requires some means for aligning timing mark 14 to the start of one of the filter segments. This alignment is typically a costly and error prone mechanical adjustment that accounts for angular differences between motor 16 and the mechanical mounting of filter segments R, G, and B. Of course, electrical or mechanical delays associated with sensor 12 further contribute to alignment errors.
The accumulated angular errors open the possibility of synchronization errors between the red, green, and blue image data to the angular position of color wheel 18, a possibility that prior workers avoided by building a timing duty cycle into the display controller electronics. The timing duty cycle provides for driving display device 26 with red, green, and blue image data for only a portion of the time when light beam 22 is propagating through each of respective filter segments R, G, and B, thereby preventing illuminating display device 26 with an improper color. Unfortunately, the timing cycle reduces the total amount of illumination available for displaying each color and, therefore, reduces the brightness of the resultant displayed color image.
Some attempts have been made to minimize color wheel synchronization errors. One example is described in copending U.S. patent application Ser. No. 09/354,998, filed Jul. 16, 1999, for LIGHT GUIDE FOR USE IN A COLOR WHEEL SYNCHRONIZATION APPARATUS AND METHOD, which is assigned to the assignee of this application. In this device, to achieve form factor goals and increase output lumens, light passes through a light integrating device, such as a light integrator tunnel, which minimizes or eliminates extra unused light downstream of the color wheel. A light guide is provided to transmit light propagated through the color wheel to a photo detector that detects a particular color or colors of light to provide an accurate index mark signal to a display controller to ensure that the appropriate red, green, and blue image data are properly synchronized with the associated color wheel filter segments. The integrator tunnel is adjacent to the light guide and provides uniform, high intensity light from the color wheel to a display device, such as a DMD.
The photo detector senses otherwise wasted light rays as sensing rays that are reflected by a light source reflector and propagated through the color wheel at predetermined locations adjacent to the integrator tunnel. To collect some of the sensing rays, the light guide is preferably bonded to the integrator tunnel. The light guide is formed from solid glass and is in the form of a solid glass rod. It operates by total internal reflection (xe2x80x9cTIRxe2x80x9d), and has a slanted output end that reflects the sensing rays toward a photo detector on a circuit board. The light guide is formed by an extrusion process and is made of solid glass. The inner surface of the light guide must be highly polished and coated to ensure that all of the light rays are collected and reflected toward the photo detector.
The light guide requires extensive manufacturing and assembly steps which include extruding it into shape and then performing post-extruding operations such as polishing and coating the inner surface. The light guide must then be bonded to the integrator tunnel in a very precise manner. These manufacturing steps make production of the light guide costly and time consuming. Additionally, the solid glass construction of the light guide adds unnecessary weight to the projection system.
It is desirable to have a projection system having a color wheel synchronization technique that circumvents the above-described problems and that is more cost effective to produce and that is light-weight.
An object of the present invention is to provide an apparatus for detecting an angular position of a color wheel in an FSC display system.
Another object of the present invention is to provide a color wheel synchronization system for a multimedia projector that is light weight, simple in design and construction, and less costly to manufacture.
A multimedia projector employing a color wheel in an FSC display system employs a light guide to transmit light propagated through the color wheel to a photo detector that detects a particular color or colors of light to provide an accurate index mark signal to a display controller to ensure that the appropriate red, green, and blue image data are properly synchronized with the associated color wheel filter segments. An integrator tunnel adjacent to the light guide provides uniform, high intensity light from the color wheel to a display device, such as a DMD.
The photo detector senses otherwise wasted light rays as sensing rays that are reflected by a light source reflector and propagated through a color wheel at predetermined locations adjacent to the integrator tunnel. To collect some of the sensing rays, the light guide is preferably formed in the housing or shroud of the motor that rotates the color wheel. The light guide is formed by an aperture or opening in the shroud that allows the light rays to pass through and reflect from a reflective surface toward the photo detector on a circuit board.
Since the color wheel rotates its filter segments from the light guide toward the integrator tunnel the photo detector senses color changes slightly before they are received by the DMD. This is advantageous because the resulting predictive timing is consistent from one projector to the next, thereby eliminating timing adjustments.
Forming the light guide integral with the motor shroud reduces the number of parts and eliminates costly and time consuming manufacturing steps, as well as reducing the weight of the projection system.
Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof that proceed with reference to the accompanying drawings.
Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof which proceeds with reference to the accompanying drawings.